The cemented carbide has conventionally been used for cutting steel and cast iron. The cemented carbide is a WC—Co alloy or a WC—Co alloy to which a carbonitride of Ti, Ta, or Nb is added. It has a problem, however, that the temperature of the edge of the cutting tool becomes a high temperature of 800° C. or more in a cutting process, which causes the edge to be plastically deformed due to the heat in the cutting process and makes it more likely that flank wear increases.
Thus, in order to improve the cutting characteristics of the cutting tool under a high temperature condition, a coated cutting tool has been used that is formed of the above-described cemented-carbide base material and a coating formed on the surface of the cemented-carbide base material. The coating is formed of a single layer or a composite layer of a carbide, a nitride, or a carbonitride of a group IVa metal in the periodic table (such as TiC, TiN, or TiCN) or a hard ceramic such as Al2O3. These coatings are formed by means of chemical vapor deposition such as CVD or physical vapor deposition such as ion plating or ion sputtering. A coating formed by means of CVD or the like has an advantage that its strength of adhesion to the cemented-carbide base material is very high and its wear resistance is highly excellent.
Meanwhile, CVD deposits the coating in a high temperature state of approximately 1000° C., and therefore, when the deposited coating is cooled to room temperature, tensile stress remains in the coating due to a difference in thermal expansion coefficient between the cemented-carbide base material and the coating. Consequently, if a crack opens in the surface of the coating in the cutting process, the tensile stress causes the crack to propagate, resulting in dropping off or chipping of the coating. It is noted that the cemented-carbide base material has a thermal expansion coefficient of approximately 5.1×10−6 K−1, and a coating made of TiN has a thermal expansion coefficient of approximately 9.2×10−6 K−1. Further, a coating made of TiC has a thermal expansion coefficient of approximately 7.6×10−6 K−1 and a coating made of Al2O3 has a thermal expansion coefficient of approximately 8.5×10−6 K−1.
Because of the recent demands for higher cutting speed and higher cutting efficiency, the coating tends to be made thicker. It is therefore necessary to further improve the adhesive strength between the cemented-carbide base material and the coating. The cutting tools that are now generally used have a thickness of the coating of about a few μm to ten and several μm. While the wear resistance is improved as the thickness of the coating is made larger, an excessively large thickness of the coating may make it more likely that an abnormal damage occurs to the tool, or the tensile stress resulting from a difference in thermal expansion efficiency may make it more likely that an abnormal damage occurs to the cutting tool. If the coating accordingly peels off from the base material, abnormal wear will occur which shortens the life of the cutting tool or deteriorates the fracture resistance.
An attempt has also been made to improve the fracture resistance by honing the edge after the base material is mechanically polished. If the surface of the base material is polished, however, a crack may occur to the interface between hard phases or the interface between a hard phase and a binder phase, or a polishing swarf may stick, which causes the adhesive strength of the coating to deteriorate. Peeling of the coating from the base material will lead to an unexpected fracture and a shortened life of the cutting tool.
In view of the above, various attempts have been made to improve the adhesive strength of the coating. For example, according to Japanese Patent Laying-Open No. 2000-212743 (PTL 1), electrolytic polishing, rather than mechanical polishing, is performed on the surface of the base material to thereby eliminate cracks in grains of the hard phase due to mechanical polishing and improve the strength of adhesion of the coating to the base material.
Japanese Patent Laying-Open No. 2008-238392 (PTL 2) discloses a technique of controlling the direction of crystal growth of the coating, according to which shot blasting is performed after brush polishing to thereby smooth a hard phase portion of the base material surface, and then a binder phase portion is removed to form a depression. The coating can thus be formed to control the direction of crystal growth of the columnar structure in the coating and enhance the toughness of the coating.